I dont know if this has been done before, but basically I have a 22mm to 15mm union, with a 22mm chamber and a ball valve for safety, then i have a piece of 15mm pipe with one end blocked off and a rubber o ring glued to the blocked end, this pushes into the union and can move back into the chamber but not out of the front, an inch infront of the o ring i drilled 4 holes in the 15mm pipe which will let air out When you fill the chamber with air the o ring seals against the front of the union preventing air from getting out but when you pull the 15mm pipe back an inch it exposes the 4 holes and air is let out shooting the projectile, then the 15mm pipe moves forward again sealing the barrel ready to fire the next shot. I am having a bit of trouble with the seal at the moment but once I get that fixed I will make a hopper with a t on the end that will slide over the barrel so that when the barrel is forward the hole is open and a projectile can fall into the barrel and when you pull the barrel back to fire it blocks the hole preventing another projectile from loading untill the barrel is forward again.

You can hear that it isnt sealing properly in the video, but most of the sounds are quite hard to hear because I left testament on in the background

Used a bit of rigid plastic tube sealed at one end with hot melt glue and with a slot carved into it just after the blockage.

It sealed in a steel broomhandle (pumped to about 60psi) using a garden hose fitting, black electrical tape, grease and a bit of cord to stop it flying out.

Forcing the plastic tube into the broomhandle let air blast through and it was used to launch pens, pencils and a thing which looked like a small rifle grenade if rifle grenades had pointy noses made of hot glue and tips of steel nails.

Range was some few tens of feet because the barrel was only about 8" long.

It's like a manually operated poppet valve which means it doesn't like high pressure and/or high flow.

It does work better at lower pressures because it is easier to seal at low pressure and its easier to push back lol. When time and money eventually allow it i want to use the same idea to make a gatling gun, but instead of pushing it back by hand the barrels will push it back as they go past.